This invention relates generally to plate fin and tube heat exchangers and methods for manufacturing heat exchangers of that type. Specifically, the invention relates to a method of manufacturing a plate fin and tube heat exchanger that has multiple rows of tubes and a curved face as well as the heat exchanger so manufactured.
FIGS. 1A, 1B, and 1C provide illustrations to assist in defining terms used in the text of this disclosure. The figures show schematically arrangements of tubes and plate fins that may be encountered in plate fin and tube type heat exchangers. FIG. 1A shows a number of tubes 01 arranged in a single row 02 and passing through stacked fins 03, each of which has a single row of holes to accommodate the single row of tubes. This is the arrangement of tubes and fins in a single row tube, single row plate fin type plate fin and tube heat exchanger. FIG. 1B depicts a number of tubes 01 arranged in two rows 02. Each of plate fins 03 has a single row of holes so that two stacks of plate fins are required for the two rows of tubes. This is the arrangement of tubes and fins in a multiple row tube, single row plate fin type plate fin and tube heat exchanger. FIG. 1C depicts a number of tubes 01 arranged in two rows 02. Each multiple row plate fin 04 has two rows of holes to accommodate the two rows of tubes so that only a single stack of plate fins is required for the two rows of tubes. This is the arrangement of tubes and fins in a multiple row tube, multiple row plate fin type of plate fin and tube heat exchanger.
In manufacturing a typical plate fin and tube heat exchanger, such as may be used in an air conditioning or refrigeration system or in an engine cooling system, U-shaped or hairpin tubes are inserted into holes in the fins and tubesheets until the open ends of the hairpin tubes protrude beyond one of the tubesheets. The walls of the tubes are then expanded radially, using a tube expander, to make firm contact between the fins and the tubes and tubesheets to ensure good heat transfer and structural strength and rigidity. The open ends of the hairpin tube legs are also expanded radially to a greater diameter than the remainder of the tube to form a bell or socket. Short U-tubes, or return bends, nipple connections from a header or a combination of return bends and header nipples are then inserted into the belled ends and secured by a suitable process such as welding, brazing or soldering to form a closed fluid flow path or paths through the heat exchanger. Some plate fin and tube heat exchangers may not use hairpin tubes but are comprised of single tubes each making a single pass through the plate fin stack. This may be the case, for example, when it is desired to have a relatively large number of separate flow paths, or circuits, through the heat exchanger.
The above process works well in manufacturing heat exchangers having but a single row of tubes or heat exchangers that have flat faces, i.e. where the finned region of the tubes remains straight in the completed heat exchanger. Many heat exchanger designs, however, call for multiple rows of tubes in order to achieve sufficient heat transfer area, and thus adequate heat transfer performance, in the heat exchanger within the constraints of overall size limitations. In addition, it is not unusual for other design considerations to call for a heat exchanger having other than a flat face. An example of such a consideration would be where the heat exchanger must have a certain face area, to satisfy heat transfer requirements, yet must fit inside an enclosure that does not have sufficient room to accommodate a flat-faced heat exchanger of the requisite size. In such a situation, it is common to configure the heat exchanger with one or more curves in its face to reduce one or more of its maximum dimensions so as to be able to fit into a given enclosure.
In plate fin and tube heat exchangers having multiple rows of tubes, it is desirable that a single plate fin be configured to have as many of the rows of tubes laced through it as possible. This configuration is desirable not only for ease of assembly but also because using multiple row plate fins prevents relative motion between the rows of tubes and contributes to the rigidity and strength of the completed heat exchanger.
To manufacture a plate fin and tube heat exchanger having a curved face, the tubes must be bent into the desired curved shape. If hairpin tubes are used, the hairpin legs are usually bent in the plane in which the hairpin bend lies rather than in a plane perpendicular to the plane of the bend so that the bend is not distorted and so that the ends of the hairpins remain even.
In a multiple tube row heat exchanger having a curved face, the tubes in a row on the inside of the curve must have a lesser radius of curvature than those on the outside in order for the rows to remain parallel after bending. As a result, a point on one tube that is directly opposite a point on an adjacent tube in a different row before bending will not be opposite that same point after bending. If a curved face multiple tube row heat exchanger were to be constructed by assembling multiple row fins on to tubes, then expanding the tubes and thus fixing the fins to the tubes, then bending the tubes to the desired curve, the relative motion between points on adjacent tubes in different rows would distort and probably tear the fin material along the portion of the tubes that are curved.
One method of preventing distortion of the fins is to bend the tubes before expanding them. Before expansion the fins can be made so that there can be slippage between the tubes and the fins, thus avoiding fin distortion and tearing.
However, the usual method of expanding tubes in a heat exchanger is by driving an expansion tool (known in the industry as a "bullet") attached to the end of a rod through the tube. This method is not suitable for expanding curved tubes. There are methods of expanding a curved tube, such as by forcing a round ball through the curved tube by fluid pressure, but such methods possess serious drawbacks in time required, equipment complexity and scrap rates.
What is needed is a method of making a multiple tube row, curved face plate fin and tube type heat exchanger that allows the use of conventional rod and "bullet" tube expansion machines and yet will produce a heat exchanger with adequate rigidity and strength.